feat(storage): add writer-exclusive JSONL tail repair

[XingYu-Zhong]

Summary

• Rebuilds the useful JSONL tail-repair logic from feat(storage): add guarded JSONL tail repair #951 on the current develop branch.
• Repairs only a malformed final record, with bounded reads, a caller-controlled durable backup, and mandatory writer exclusion.

Changes

• Requires a runExclusive(path, operation) maintenance runner around the complete inspect/backup/replace/verify transaction.
• Scans JSONL incrementally with hard byte/record caps and preserves an exact SHA-256-addressed backup snapshot before mutation.
• Uses a synced exclusive temp file, atomic replacement, stability checks, and an original-inode guard for already-open writers.
• Returns changed/a retained conflict path instead of choosing a version when safety cannot be proven.

All writers for the repaired path must cooperate with the same maintenance runner; the service fails immediately when no runner is supplied.

Tests

• Targeted repair/concurrency tests (8 passed).
• Full Kun suite (2148 passed, 1 skipped).
• Kun typecheck/build, targeted ESLint, and git diff --check.

Original contribution and repair direction: @luoye520ww in #951.

Replaces #951.